1. Field of the Invention
The invention relates to an alkali-free aluminoborosilicate glass. The invention also relates to uses of this glass.
2. Background of the Invention
High requirements are made of glasses for applications as substrates in flat-panel liquid-crystal (or expressed differently: liquid crystal) display technology, for example in TN (twisted nematic)/STN (supertwisted nematic, or expressed differently: super twisted) displays, active matrix liquid crystal displays (AMLCDs), thin film transistors (TFTs) or plasma addressed liquid crystals (PALCs). Besides high thermal shock resistance and good resistance to the aggressive chemicals employed in the process for the production of flat-panel screens, the glasses should have high transparency over a broad spectral range (VIS, UV) and, in order to save weight, a low density. Use as substrate material for integrated semiconductor circuits, for example in TFT displays (xe2x80x9cchip on glassxe2x80x9d) in addition requires thermal matching to the thin-film material silicon which is usually deposited on the glass substrate in the form of amorphous silicon (a-Si) at low temperatures of up to 300xc2x0 C. The amorphous silicon is partially recrystallized by subsequent heat treatment at temperatures of about 600xc2x0 C. Owing to the a-Si fractions, the resulting, partially crystalline poly-Si layer is characterized by a thermal expansion coefficient of xcex120/300≅3.7xc3x9710xe2x88x926/K. Depending on the a-Si/poly-Si ratio, the thermal expansion coefficient xcex120/300 may vary between 2.9xc3x9710xe2x88x926/K and 4.2xc3x9710xe2x88x926/K. When substantially crystalline Si layers are generated by high temperature treatments above 700xc2x0 C. or direct deposition by CVD processes, which is likewise desired in thin-film photovoltaics, a substrate is required which has a significantly reduced thermal expansion of 3.2xc3x9710xe2x88x926/K or less. In addition, applications in display and photovoltaics technology require the absence of alkali metal ions. Sodium oxide levels of less than 1000 ppm (parts per million) as a result of production can be tolerated in view of the generally xe2x80x9cpoisoningxe2x80x9d action due to diffusion of Na+ into the semiconductor layer.
It should be possible to produce suitable glasses economically on a large industrial scale in adequate quality (no bubbles, knots, inclusions), for example in a float plant or by drawing methods. In particular, the production of thin ( less than 1 mm) streak-free substrates with low surface undulation by drawing methods requires high devitrification stability of the glasses. In order to counter compaction of the substrate during production, in particular in the case of TFT displays, which has a disadvantageous effect on the semiconductor microstructure, the glass needs to have a suitable temperature-dependent viscosity characteristic line: with respect to thermal process and shape stability, it should have a sufficiently high glass transition temperature, i.e. Tg greater than 700xc2x0 C., while on the other hand not having excessively high melting and processing (VA) temperature, i.e. a VA of xe2x89xa61350xc2x0 C.
The requirements of glass substrates for LCD display technology or thin-film photovoltaics technology are also described in xe2x80x9cGlass substrates for AMLCD applications: properties and implicationsxe2x80x9d by J. C. Lapp, SPIE Proceedings, Vol. 3014, invited paper (1997), and in xe2x80x9cPhotovoltaikxe2x80x94Strom aus der Sonnexe2x80x9d by J. Schmid, Verlag C. F. Muller, Heidelberg 1994, respectively.
The abovementioned requirement profile is fulfilled best by alkaline earth metal aluminoborosilicate glasses. However, the known display or solar cell substrate glasses described in the following publications still have disadvantages and do not meet the full list of requirements.
Some documents describe glasses containing relatively little or no BaO, e.g. European Patent No. 714 862 B1, International Patent Application No. 98/27019, Japanese Patent No. 10-72237 A and European Patent No. 510 544 B1. Glasses of this type, in-particular those having low coefficients of thermal expansion, i.e. low RO content and high network former content, are very susceptible to crystallization. Furthermore, most of the glasses, in particular in EP 714862 B1 and JP 10-72237 A, have high temperatures at a viscosity of 102 dPas.
However, the preparation of display glasses having high levels of the heavy alkaline earth metal oxides BaO and/or SrO is likewise associated with great difficulties owing to the poor meltability of the glasses. In addition, glasses of this type, as described, for example, in DE 37 30 410 A1, U.S. Pat. Nos. 5,116,789, 5,116,787, EP 341 313 B1, EP 510 543 B1 and JP 9-100135 A, have an undesirably high density.
Even glasses having relatively low SrO contents in combination with moderate to high BaO levels have unfavorable viscosity characteristic lines with respect to their meltability, for example the glasses described in JP 9-169538 A, WO 97/11920 and JP 4-160030 A.
Glasses having relatively high levels of light alkaline earth metal oxides, in particular MgO, as described, for example, in JP 9-156953 A, JP 8-295530 A, JP 9-48632 A and DE 197 39 912 C1, exhibit good meltability and have a low density. However, they do not meet all requirements made of display and solar cell substrates with regard to chemical resistance, in particular to buffered hydrofluoric acid, to crystallization stability and to heat resistance. Glasses having low boric acid contents exhibit excessively high melting-temperatures or, as a result of this, excessively high viscosities at the melt and processing temperatures required for processes involving these glasses. This applies to the glasses of JP 10-45422 A and JP 9-263421 A.
Moreover, glasses of this type have a high devitrification tendency when combined with low BaO contents.
In contrast, glasses having high boric acid contents, as described, for example, in U.S. Pat. No. 4,824,808, have insufficient heat resistance and chemical resistance, in particular to hydrochloric acid solutions.
Glasses having a relatively low SiO2 content do not have sufficiently high chemical resistance either, in particular when they contain relatively large amounts of B2O3 and/or MgO and are low in alkaline earth metals. This applies to the glasses of WO 97/11919 and EP 672 629 A2. The relatively SiO2-rich variants of the latter document have only low Al2O3 levels, which is disadvantageous for the crystallization behavior.
The glasses described in Japanese Patent No. 9-12333 A for hard disks are comparatively low in Al2O3 or B2O3, the latter merely being optional. The glasses have high alkaline earth metal oxide contents and have high thermal expansion, which makes them unsuitable for use in LCD or PV technology.
Federal Republic of Germany Patent No. 196 17 344 C1 (U.S. Pat. No. 5,908,703) and Federal Republic of Germany Patent No. 196 03 698 C1 (U.S. Pat. No. 5,770,535) by the Applicant disclose alkali-free, tin oxide-containing glasses having a coefficient of thermal expansion xcex120/500 of about 3.7xc3x9710xe2x88x926/K and very good chemical resistance. They are suitable for use in display technology. However, since they must contain ZnO, they are not ideal, in particular for processing in a float plant.
In particular at higher ZnO contents ( greater than 1.5% by weight), there is a risk of formation of ZnO coatings on the glass surface by evaporation and subsequent condensation in the hot-shaping range.
Federal Republic of Germany Patent No. 196 01 022 A1 describes glasses which are selected from a very wide composition range and which must contain ZrO2 and SnO. The glasses, which, according to the examples, contain SrO, tend to exhibit glass defects because of their ZrO2 level.
Federal Republic of Germany Patent No. 42 13 579 A1 describes glasses for TFT applications having a coefficient of thermal expansion xcex120/300 Of  less than 5.5xc3x9710xe2x88x926/K, according to the examples of xe2x89xa74.0xc3x9710xe2x88x926/K. These glasses which have relatively high B2O3 levels and relatively low SiO2 contents do not have a high chemical resistance, in particular to diluted hydrochloric acid.
U.S. Pat. No. 5,374,595 describes glasses having coefficients of thermal expansion of between 3.2xc3x9710xe2x88x926/K and 4.6xc3x9710xe2x88x926/K. The glasses which, as the examples illustrate, have high BaO contents, are relatively heavy and exhibit poor meltability and a thermal expansion which is not ideally matched to substantially crystalline Si.
In the unexamined Japanese publication nos. 10-25132 A, 10-114538 A, 10-130034 A, 10-59741 A, 10-324526 A, 11-43350 A, 11-49520 A, 10-231139 A and 10-139467 A, mention is made of very wide composition ranges for display glasses, which can be varied by means of many optional components and which are admixed with one or more specific refining agents in each case. However, these documents do not indicate how glasses having the complete requirement profile described above can be obtained in a specific manner.
It is an object of the present invention to provide glasses which meet said complex requirement profile with respect to the physical and chemical properties which is imposed on glass substrates for liquid-crystal displays, and for thin-film solar cells, in particular on the basis of xcexcc-Si, glasses which have high heat resistance, a favorable processing range and sufficient devitrification stability.
The invention teaches that this object can be accomplished by an aluminoborosilicate glass having a coefficient of thermal expansion xcex120/300 of between 2.8xc3x9710xe2x88x926/K and 3.8xc3x9710xe2x88x926/K, which has the following composition (in % by weight, based on oxide): silicon dioxide (SiO2)xe2x80x94from somewhat greater than 58% to 65% ( greater than 58%-65%); boric oxide (B2O3)xe2x80x94from somewhat greater than 6% to 10.5% ( greater than 6%-10.5%); aluminum oxide (Al2O3)xe2x88x92from somewhat greater than 14% to 25 ( greater than 14%-25%); magnesium oxide (MgO)xe2x88x92from 0% to somewhat less than 3% (0%- less than 3%); calcium oxide (CaO)xe2x88x92from 0% to 9% (0%-9%); barium oxide (BaO)xe2x88x92from somewhat less than 3% to 8% ( greater than 3%-8%); with magnesium oxide (MgO)+calcium oxide (Cao)+barium oxide (BaO)xe2x88x92from 8% to 18% (8%-18%); and zinc oxide (ZnO)xe2x88x92from 0%xe2x88x92to somewhat less than 2% (0%- less than 2%).
The glass contains between  greater than 58 and 65% by weight of SiO2. At lower contents, the chemical resistance is impaired, while at higher levels, the thermal expansion is too low and the crystallization tendency of the glass increases.
The glass contains relatively high levels of Al2O3, i.e.  greater than 14-25% by weight, preferably 18-25% by weight. Such Al2O3 levels are favorable for the crystallization stability of the glass and have a positive effect on its heat resistance without excessively increasing the processing temperature. Preference is given to a content of more than 18% by weight, particularly preferably of at least 20.5% by weight, most preferably of at least 21.5% by weight, of Al2O3.
The B2O3 content is  greater than 6-10.5% by weight, preferably  greater than 8-10.5% by weight. The B2O3 content is restricted to the maximum content specified in order to achieve a high glass transition temperature Tg. Higher contents would also impair the chemical resistance to hydrochloric acid solutions. The minimum B2O3 content specified serves to ensure that the glass has good meltability and good crystallization stability. The network-forming components Al2O3 and B2O3 are preferably present at mutually dependent minimum levels, ensuring a preferred content of the network formers SiO2, Al2O3 and B2O3. For example, in the case of a B2O3 content of  greater than 6-10.5% by weight, the minimum Al2O3 content is preferably  greater than 18% by weight, and in the case of an Al2O3 content of  greater than 14-25% by weight, the minimum B2O3 content is preferably  greater than 8% by weight. Preferably, in particular to achieve very low thermal expansion coefficients of up to 3.6xc3x9710xe2x88x926/K, the sum of SiO2, B2O3 and Al2O3 is more than 84% by weight.
An essential glass component are the network-modifying alkaline earth metal oxides. In particular by varying their levels, a coefficient of thermal expansion xcex120/300 of between 2.8xc3x9710xe2x88x926/K and 3.8xc3x9710xe2x88x926/K is achieved within a sum content of 8-18% by weight in total. The maximum sum of alkaline earth metal oxides is preferably 15% by weight, particularly preferably 12% by weight. The latter upper limit is in particular advantageous for obtaining glasses having very low xcex120/300 less than 3.2xc3x9710xe2x88x926/K) coefficients of thermal expansion. BaO is always present, while MgO and CaO are optional components. Preferably at least two alkaline earth metals are present, particularly preferably all three alkaline earth metals mentioned are present.
The BaO content is between  greater than 3 and 8% by weight. These relatively high BaO levels were found to ensure a sufficient crystallization stability for the various flat glass production processes such as float methods and the various drawing methods, in particular in the case of low-expansion glass variants having quite high levels of network-forming components and thus a crystallization tendency which is in principle rather high. The maximum BaO content is preferably limited to 5% by weight, particularly preferably 4% by weight, most preferably  less than 4% by weight, which has a positive effect on the desired low density of the glasses.
SrO is omitted, thus maintaining low melting and hot shaping temperatures and a low density of the glass. However, minor amounts, i.e. less than 0.1% by weight, introduced into the glass melt as impurities of the batch raw materials, can be tolerated.
The glasses may furthermore contain up to 9% by weight of CaO. Higher levels would lead to an excessive increase in thermal expansion and an increase in crystallization tendency. The glass preferably contains at least 2% by weight of CaO.
The glass may also contain up to  less than 3% by weight of MgO. Relatively high levels are beneficial for a low density and a low processing temperature, whereas relatively low levels are favorable with regard to the chemical resistance of the glass, in particular to buffered hydrofluoric acid, and its devitrification stability.
The glasses may furthermore contain up to  less than 2% by weight of ZnO. ZnO has an effect on the viscosity characteristic line which is similar to that of boric acid, has a structure-loosening function and has less effect on the thermal expansion than the alkaline earth metal oxides. The maximum ZnO level is preferably limited to 1.5% by weight, particularly preferably 1.0% by weight, most preferably less than 1% by weight, in particular when the glass is processed by the float method. Higher levels would increase the risk of unwanted ZnO coatings on the glass surface which may form by evaporation and subsequent condensation in the hot-shaping range. The presence of at least 0.1% by weight is preferred, as the addition of ZnO, even in small amounts, leads to an increase in devitrification stability.
The glass is alkali-free. The term xe2x80x9calkali-freexe2x80x9d as used herein means that it is essentially free from alkali metal oxides, although it can contain impurities of less than 1000 ppm (parts per million).
The glass may contain up to 2% by weight of ZrO2+TiO2, where both the TiO2 content and the ZrO2 content can each be up to 2% by weight. ZrO2 advantageously increases the heat resistance of the glass. Owing to its low solubility, ZrO2 does, however, increase the risk of ZrO2-containing melt relicts, so-called zirconium nests, in the glass. ZrO2 is therefore preferably omitted. Low ZrO2 contents originating from corrosion of zirconium-containing trough material are unproblematic. TiO2 advantageously reduces the solarization tendency, i.e. the reduction in transmission in the visible wavelength region because of UV-VIS radiation. At contents of greater than 2% by weight, color casts can occur due to complex formation with Fe3+ ions which are present in the glass at low levels as a result of impurities of the raw materials employed.
The glass may contain conventional refining agents in the usual amounts: it may thus contain up to 1.5% by weight of As2O3, Sb2O3, SnO2, CeO2, C1xe2x88x92 (for example in the form of BaCl2), Fxe2x88x92 (for example in the form of CaF2) and/or SO42xe2x88x92 (for example in the form of BaSO4). The sum of the refining agents should, however, not exceed 1.5% by weight. If the refining agents As2O3 and Sb2O3 are omitted, this glass can be processed not only using a variety of drawing methods, but also by the float method.
For example with regard to easy batch preparation, it is advantageous to be able to omit both ZrO2 and SnO2 and still obtain glasses having the property profile mentioned above, in particular having high heat and chemical resistance and low crystallization tendency.
The above-discussed embodiments of the present invention will be described further hereinbelow. When the word xe2x80x9cinventionxe2x80x9d is used in this specification, the word xe2x80x9cinventionxe2x80x9d includes xe2x80x9cinventionsxe2x80x9d, that is the plural of xe2x80x9cinventionxe2x80x9d. By stating xe2x80x9cinventionxe2x80x9d, the Applicants do not in any way admit that the present application does not include more than one patentably and non-obviously distinct invention, and maintains that this application may include more than one patentably and non-obviously distinct invention. The Applicants hereby assert that the disclosure of this application may include more than one invention, and, in the event that there is more than one invention, that these inventions may be patentable and non-obvious one with respect to the other.